The metals in the context of the invention find use as “plant components” in e.g. optionally heated pipes for the transport of polycarbonate melts, tube bundles or the internal surfaces of heat exchangers, reaction vessels or filtration apparatus or other melt-conveying sections of extruders and pumps. The polycarbonate thus produced is especially suitable for high-quality injection molded articles, particularly those in which high transparency and a low YI (yellowness index) are important, such as e.g. optical data storage media, diffusers or panes for the automotive sector in general. The polycarbonate granules or injection moldings produced with the aid of these plant components have a low content of fluorescent particles.
The production of polycarbonate generally takes place in plant components, i.e. pipes, reaction vessels etc., made of steel or of special steel alloys. The polycarbonate is produced, transported, evaporated, isolated or further processed in equipment that includes these components.
It has now been found that metal surfaces that come into contact with polycarbonate melts can have a deleterious effect on product quality.
Thus, it has been found that polycarbonate produced in these plant components exhibits a certain contamination with fluorescent particles, which is disadvantageous for the quality of injection moldings, extrudates and sheets/films produced from this material.
High, residence times of polycarbonate melts in these plant components are particularly disadvantageous to the product quality and increase the intensity of fluorescence in the material. Polycarbonate melts produced by the melt transesterification process, for example, are in contact with metal surfaces for a prolonged period. This process in particular is therefore critical in terms of the formation of fluorescent particles, which reduce the product quality.
Not only high residence times but also the area of the metal surface coming into contact with the polymer melt plays a serious role. Thus, for example, plant components for the filtration of polycarbonate melts may be critical for product quality since the polycarbonate is exposed to a large area of metal surface. Surface quality is also a factor to be considered, a polished surface is more advantageous than a non-polished.
A process for the treatment of metallic plant components that come into contact with polycarbonate melts has therefore been developed. This process enables a polycarbonate to be produced with a lower content of fluorescent particles, thus distinctly improving the product quality.
In addition to use in optical data storage media, this polycarbonate is also highly suitable for the production of moldings from the automotive glazing sector, such as e.g. diffusers. This polycarbonate is particularly advantageous in those applications in which a good surface quality is required.
It is known that the treatment of steels has an effect on susceptibility to corrosion. Passivation methods are described e.g. by Asami et al. in “Surface and Interface Analysis”, 2004, 36(3), 220-224 or by Virtanen et al. in “Material Science Forum” 1995, 185-188 (Passivation of Metals and Semiconductors), 965-974.
None of the publications cited above relates to the treatment of metals that come into contact with polycarbonate melts. The problems of corrosion are not comparable with the requirements of polycarbonate production. The solutions found were not therefore obvious.
It is known that discoloration of the polycarbonate can result from ferrous surfaces in contact with a polycarbonate melt. This is described e.g. in JP 02233733. To solve this problem, low-iron alloys, such as e.g. alloy 59, have been used in combination with polycarbonate melts. This is described e.g. in EP 1222231 B1, WO 2000064554, WO 9954381, WO 2000007684 or in JP 07171873. Apart from the fact that constructions in alloy/Hastelloy are very expensive, so that only partial areas of a plant may ever be built from these materials, none of these publications describes specific treatments of the alloys by means of which the formation of fluorescent particles in polycarbonate melts may be avoided.
In EP 0 410 425 A2, injection moldings made of polycarbonate with a low dust content are described. This is achieved in that certain machine components of the injection molding machine, such as e.g. the screw elements, consist of certain corrosion-resistant materials, such as e.g. Hastelloy. No connection with fluorescent bodies is reported and no treatment method is described.
In JP 2002 12 64 25, a melt filtration plant is described consisting of a stainless steel filter for the reduction of gel particles. No connection with fluorescent bodies is reported and no treatment method of the working surfaces is described.
In EP-A 1156071, a passivation of filter media with weak acids, such as e.g. phenol, is described. As a result, an improvement in the yellowness index of the filtered material is achieved. Fluorescent particles are not described. This method is unsuitable for the reduction of fluorescent particles. In this publication a heat treatment in nitrogen is described, whereas the present invention treats oxidatively.
In U.S. Pat. No. 4,518,440, a process for the passivation of metal surfaces with mineral acids is described. These surfaces, which have a specific chromium oxide and iron oxide content after this process, are generally passivated by the method described. No effects on the formation of fluorescent particles in polycarbonate melts are described. Comparative tests in which polycarbonate melts were treated with iron oxide and/or chromium oxide show that a distinct increase in fluorescence occurs in polycarbonate melts. Taking these results into account, this passivation method is unsuitable for polycarbonate melts.
In JP-4-161210, the coating with chromium oxide of filter media used to filter polymer melts is described. This coating is achieved by a wet chemical process. The effects on the formation of fluorescent particles are not described. A reduction in gel particles is achieved by this method. In contrast, the passivation is achieved here primarily by a thermal process. The passivation according to the invention described here is clearly more effective in terms of the formation of fluorescent particles in PC melts.
In JP 2003048975, the production of polycarbonate in the melt transesterification process is described. In this process, the molten raw materials are subjected to filtration through a passivated filter. This passivation takes place with acid and/or tempering under an inert gas atmosphere. There is no mention of the formation of fluorescent particles. The method described deviates from the method disclosed here since oxidative passivation methods are used here. The filter described in JP 2003048975 is not used for the melt filtration of polycarbonate.
None of the above-mentioned publications deals with the content of fluorescent impurities in polycarbonate. The methods mentioned above do not lead to an effective reduction in fluorescent particles in polycarbonate.